Publication date: 18th June 2020
I) Dynamic Disorder and Urbach Tails in CsPbBr3 from First Principles
The characteristic energy scale of the low energy absorption tail, i.e. the Urbach energy of an absorption spectrum is known to be an excellent measure of disorder in a material and has long been linked with photovoltaic efficiency. We have developed the theoretical tools required to calculate finite temperature Urbach tails that arise from dynamic disorder from first principles. We combine ab-initio Density Functional Theory (DFT) with a finite differences approach to electron-phonon coupling to model the temperature dependance of the Urbach energy in an archetype inorganic perovskite CsPbBr3. We have also developed the capacity to save the wavefunctions of the 'tail states' that contribute to the Urbach region of the absorption in order to study their localisation properties. We find that at the PBE level of theory with gamma-point phonons, the zero point quantum motion of the nuclei appear to contribute to an Urbach tail, potentially redefining the origin of the 'static' Urbach energy.
II) Modelling Ultrafast Complex Refractive Index Changes from Transient Absorption Spectroscopy
Transient absorption (TA) spectroscopy is a proxy for changes in the extinction coefficient, i.e, the complex part of the refractive index. Large changes in the real part of the refractive index reported in metal halide perovskites [4] can obfuscate a truly quantitative analysis of the TA data. Further, large changes in the real part of the refractive index typically dominate transient reflection (TR) spectroscopy. We find that, equipped with static ellipsometric measurements, we are able to use a Kramers-Kronig constrained variational analysis to decompose transient spectra into changes in the complex refractive index. We are able to satisfactorily reproduce experimental results on CsPbBr3 using white light interferometry [4] from TA and static ellipsometric measurements alone. We are further able to understand TR spectra in GaAs.
A.A. would like to thank the Gates Cambridge Trust and the Winton Program for the Physics of Sustainability for studentship funding.
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